I keep a log book, I write the date, the muscle groups, weight used and the reps. It helps a lot, sometimes you don't remember how much weight you used for which machine, or dummbell. Also, if you use machines, I write the brand of machines, you can handle different weights on different machines. Sometimes you increase the weight in a workout and you won't remember that you did on your next workout, all I have to do is look at my log book if I don't remember what I lifted on my last workout.
